1. Field
The present invention relates generally to methods, apparatus and systems for transmitting and receiving data. For example, certain embodiments of the present invention provide methods, apparatus and systems for providing one or more additional channels in a frame structure comprising a preamble portion and a data portion. Certain embodiments of the present invention may be applied in existing and future generation digital broadcasting systems, for example systems developed by the Digital Video Broadcasting (DVB) Project and/or the Advanced Television Systems Committee (ATSC).
2. Description of the Related Art
Digital broadcasting techniques allow various types of digital content, for example video and audio data, to be distributed to end users. A number of standards have been developed for this purpose, including a family of standards developed by the ATSC organization, including standards ATSC 1.0 and ATSC 2.0. The ATSC Digital Television (DTV) Standard, described in various documents, including A/52 and A/53, available at http://www.atsc.org/, have been adopted for use in terrestrial broadcasting by various countries, including the United States, Canada and South Korea.
Recently, ATSC has begun developing a new standard, known as ATSC 3.0, for a delivery method of real-time and non-real-time television content and data to fixed and mobile devices. As part of this development, ATSC has published a Call for Proposals (CFP) document (TG3-S2 Doc. #023r20, “Call for Proposals For ATSC-3.0 PHYSICAL LAYER, A Terrestrial Broadcast Standard”, ATSC Technology Group 3 (ATSC 3.0), 26 Mar. 2013), in which a stated goal is to identify technologies that could be combined to create a new physical layer of an ATSC 3.0 Standard. It is envisaged that the ATSC 3.0 system will be designed with a layered architecture and a generalized layering model for ATSC 3.0 has been proposed. The scope of the aforementioned CFP is limited to the base layer of this model, the ATSC 3.0 Physical Layer, which corresponds to Layer 1 and 2 of the ISO/IEC 7498-1 model.
It is intended that ATSC 3.0 will not require backward compatibility with existing broadcasting systems, including ATSC 1.0 and ATSC 2.0. However, the CFP states that, wherever practicable, the standard shall utilize and reference existing standards that are found to be effective solutions to meet the requirements.
Other existing standards developed for broadcasting digital content include a family of open standards developed and maintained by the Digital Video Broadcasting (DVB) Project and published by the European Telecommunications Standards Institute (ETSI). One such standard is DVB-T2, which is described in various documents, including ETSI EN 302 755 V1.3.1, (“Digital Video Broadcasting (DVB); Frame structure channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2)”), and Technical Specification ETSI TS 102 831 V1.2.1 (“Digital Video Broadcasting (DVB); Implementation guidelines for a second generation digital terrestrial television broadcasting system (DVB-T2)”).
In DVB-T2, data is transmitted in a frame structure, which will now be briefly described with reference to FIG. 1. At the top level, the frame structure 100 consists of super-frames 101a-c, which are divided into a number of T2-frames 103a-d. Each T2-frame 103a-d is sub-divided into OFDM symbols, including a number of preamble symbols 105, 107a-c followed by a number of data symbols 109a-e. In a T2-frame 103a-d, the preamble symbols 105, 107a-c comprise a single P1 preamble symbol 105, followed by one or more P2 preamble symbols 107a-c. 
The P1 symbol 105, located at the beginning of a T2 frame 103a-d, carries 7 bits for signalling, including Si signalling used to identify the format of the P2 symbols 107a-c and S2 signalling used to signal certain basic transmission parameters. The P2 symbols 107a-c, immediately following the P1 symbol 105, are used for fine frequency and timing synchronisation and channel estimation. The P2 symbols 107a-c carry L1 signalling information, and may also carry data. The L1 signalling is divided into L1-pre signalling and L1-post signalling. The L1-pre signalling includes basic information about the T2 frame structure 100, and enables the reception and decoding of the L1-post signalling. The L1-post signalling provides sufficient information for the receiver to decode Physical Layer Pipes (PLPs) within the T2-frames, which carry data.
Service data (e.g. in the form of one or more MPEG-2 Transport Streams) may be separated into one or more data streams, which are then carried in the form of PLPs. Each PLP is a logical channel, which may carry one or multiple services.
The procedure by which information is mapped to symbols of a T2-frame 103a-d will now be briefly described.
In the following, a modulation value for one OFDM carrier during one OFDM symbol (e.g. a single constellation point) may be referred to as an OFDM cell. An OFDM cell corresponding to data (e.g. PLP data) may be referred to as a data cell and an OFDM cell corresponding to signalling may be referred to as a signalling cell. An OFDM cell corresponding to L1 signalling (including L1-pre and L1-post) may be referred to as an L1 signalling cell.
The carriers of the OFDM symbols forming a T2-frame 103a-d may be pictured as forming a grid of cells 200, as illustrated in FIG. 2. Cells of the grid 200 corresponding to different carriers of the same OFDM symbol may be arranged in the same column (e.g. with cells corresponding to carriers of increasing frequency being arranged from top to bottom), while cells of the grid 200 corresponding to carriers having the same carrier frequency of different OFDM symbols may be arranged in rows (e.g. with cells corresponding to OFDM symbols in order of time being arranged from left to right).
Information (including signalling and data) is mapped to symbols of the T2-frame 103a-d. This process may be pictured as mapping OFDM cells, including signalling cells and data cells, to cells of a grid 200 of the form described above.
For example, S1 and S2 signalling bits are mapped to cells of the P1 symbol 205. Next, L1 signalling cells are mapped to cells of the P2 symbols 207a-c in a row-wise zig-zag manner, such that L1 signalling cells are mapped to fill a row of the P2 symbols 207a-c before proceeding to the next row. After L1 signalling cells have been mapped to cells of the P2 symbols 207a-c, the remaining active cells of the P2 symbols 207a-c (e.g. excluding cells used for pilot signals), and the following data symbols 209a-e, are available for carrying PLP data in a non zig-zag manner.
In digital broadcasting systems, in order to recognize and properly decode a received signal it is necessary to first perform synchronisation. Synchronization allows the receiver to identify the presence of a frame in the received signal and to identify the beginning of the frame. In addition, the values of one or more system parameters typically need to be signalled by the transmitter to the receiver to assist the receiver in decoding the signal. Detection, synchronization and signalling information is typically provided in a preamble portion of a received signal. For example, in DVB-T2, synchronization and signalling is achieved using the P1 and P2 symbols 105, 107a-c. 
What is desired is a technique for transmitting and receiving additional data within an existing or future frame structure. For example, what is desired is a technique for providing one or more additional data channels in a frame structure in existing and future generation digital broadcasting systems, for example systems developed by the Digital Video Broadcasting (DVB) Project and/or the Advanced Television Systems Committee (ATSC) (e.g. the ATSC 3.0 Standard).